The present invention relates to a thin-film electroluminescent (TFEL) device capable of providing a full color display through the use of a hybrid structure employing stacked independently addressable subpanels, one of which includes patterned phosphor stripes.
Multi-colored TFEL devices have been proposed having one of two basic structures. In the first, an example of which is described in the aforementioned parent application of which this application is a continuation-in-part, independently addressable stacked subpanels emit light of two differing colors. Thus, varying hues in at least a portion of the visible color spectrum are available. In order to provide a full color display, it would be necessary, with this type of architecture, to have three stacked subpanels with each subpanel emitting one of the primary colors. This is impractical because of the transmission losses from the rearmost panel, and because of the parallax error inherent in such a design.
Patterned phosphor stripes arranged in side-by-side relation on a single subpanel are capable of providing a full color display. However, in order to obtain good resolution, a high pixel density must be employed. With a high pixel density, however, the area occupied by each of the phosphor stripes must be smaller, which leads to an unacceptable decrease in luminance.
Recently a hybrid structure has been proposed as shown in Roberson, U.S. Pat. No. 4,689,522, comprising two stacked subpanels forming the front and rear of a laminate structure including a solid phosphor layer and a patterned phosphor layer comprising phosphor stripes of alternate color emitting properties. The two phosphor layers are sandwiched between front and rear column electrodes and share an interiorly disposed set of row or scanning electrodes. The phosphor stripes have uniform widths to form square picture elements. The problem with the foregoing arrangement is the difficulty in driving the panel with the shared common scanning electrode. The front and rear phosphor layers are driven simultaneously and with the same field intensity generated by the common scanning electrode. This arrangement, unfortunately, does not provide for adjustments to deal with the difference in brightness levels between the front and rear phosphor layers which are needed in order to obtain the right color mix for achieving a full color display. Also the fact that the two phosphor layers are contained in a laminate structure and are unseparated except by insulating layers means that the chromaticity of the rear layer will be dependent upon the color emitting properties of the phosphor itself. Thus, a phosphor having the right color hue may not be bright enough and vice versa.